The hypothesis for the death of al the fish is that this large school seeked shelter against tidal currents in this shallow bay/harbour, and because they were with so many they used up all the oxigen in the water and sufficated.
Besides all the dead fish on the surface there is a 1 1/5 ft layer of dead fish on the bottom of the harbour as well, news reported.

I dunno they must be some dumb mother $#@!ers to sit there and die when they can just swim away lol.

What I mean is they do not think at all. They are a school of fish and all reactions are primary responces to external imput. There is no thinking involved. They have no capacity to reason.
They seeked shelter to rest, and when the oxigen levels in the water depleted all fish too far in the bay had no means of escape.
Sometimes schools can get too big for small spaces.